1. The Field of the Invention
The present invention relates generally to crank arms. More particularly, it concerns a method of manufacturing a bicycle crank arm which is sturdy, lightweight and interchangeable, for structurally linking a bicycle sprocket and a foot pedal.
2. The Background Art and Background of the Invention
The bicycle industry is a multi-million dollar industry which relies heavily on interchangeable parts to make bicycles easier to repair and thus more affordable for the average consumer. Bicycles are generally used for commuting, recreation and sport. Bicycling has become quite popular, in part because it is a means of travel which is environmentally clean and physically challenging. Of current interest are apparatus and methods of making interchangeable bicycle parts which are relatively lightweight and have a modern appearance but which also possess the necessary strength.
A bicycle is generally a vehicle having a structural steel frame, two wheels in tandem, pedals connected to a sprocket, a continuous circular chain connecting the rear wheel and the sprocket, handlebars for steering, and a saddlelike seat. A user sits on the seat and pumps the pedals to power the sprocket and hence the chain, thereby manually actuating the rear wheel and causing it to frictionally engage the ground and thus propel the bicycle. Bicycle routes include various types of terrain and conditions: asphalt streets, dirt roads, sports tracks, foothills and even mountain trails, all in conditions ranging from extreme summer heat to cold winter conditions. Bicycles must therefore not only be light enough for a user to propel with his or her own strength, they must also be strong enough to absorb the repeated impact forces imposed by unpredictable terrain, and durable enough to withstand seasonal weathering and extreme temperature change.
The bicycle frame has been the main focus of industry efforts to optimize the weight to strength ratio of the bicycle. Prior to the turn of the century, bicycle frames were made of solid steel and were called "bonebreakers" because they were so heavy and difficult to maneuver. Today, however, bicycle frames are much lighter and typically comprise integrally connected tubular steel members. Various structural designs have been developed for making bicycle frames which are lighter than their solid steel ancestors but which are strong enough to withstand the customary impact forces during use. Although there are many materials which are lighter than structural metal, the industry continues to make bicycle frames from structural steel and other metal alloys to ensure sufficient impact strength, durability and weatherability.
However, the frame is not the only focus of industry efforts to reduce the overall weight of the bicycle. For example, today's bicycle tires and inner tubes are narrower and lighter than their predecessors. Bicycle seats are smaller and lighter. The industry continually tries to optimize the weight to strength ratio of bicycles by redesigning not just the frame, but the smaller components as well. Designers also strive to develop bicycles which have a uniquely modern look by altering the configuration and structural arrangement of the frame as well as the shape, color and texture of the component bicycle parts.
Bicycle crank arms, the structural members which link the foot pedals to the sprocket, are made of solid steel or other metals even today. The bicycle crank arm is unique because it must withstand simultaneous direct applications of bending and torsional loads introduced by the foot pedal which is attached at a proximal end thereof. Moreover, for a mechanical member such as a crank arm to be interchangeable, it must comprise a material which is stiff and strong enough to avoid undue deformation or stripping at its connecting points with other mechanical members. The bicycle crank arm must also be durable enough to withstand the indirect effects of impact forces which pass through the frame.
The conventional approach to constructing bicycle crank arms typically involves the use of structural steel, aluminum, or metal alloys involving steel, titanium, magnesium and other metals. Structural steel has been used over other traditional construction materials for its hardness and superior bending and torsional strength, durability, and thermal capacity which allows it to withstand the seasonal temperature changes encountered by both summer and winter cycling. The industry continues to research and develop metal alloys in order to minimize density while maintaining strength, durability and thermal capacity to tolerate temperature change. Steel and metal alloys are therefore used in fabricating interchangeable crank arms and other structural members which must withstand bending, torsion, weathering and other wear.
Previously available bicycle crank arms thus include a single, solid metal integral member which has for decades met the needs of hardness, bending and torsion imposed during bicycle use. However, the previously available bicycle crank arms have many disadvantages. They are heavy and offer limited opportunity for the design of a modern appearance. Moreover, they rely on a single integral member to withstand both torsion and bending, which is a less efficient mechanical use of materials. Crank arms made of metal require a long time to manufacture. These and other problems, when recognized, represent needs which have been unmet in the industry until the advent of the present invention, to the knowledge of the inventor.
There is thus a need to achieve a lighter, modern looking bicycle crank arm which is still strong enough to withstand simultaneous bending and torsional loads, and durable enough to withstand the indirect effects of impact forces. There is also a need for a crank arm that is not limited to a single structural component for withstanding both bending and torsional loads, and the resulting mechanical stress and strain. Those having ordinary skill in the art will appreciate that these and other needs are met by the present invention.
The disadvantages of the prior art are overcome in the present invention by provision of a composite crank arm and a method of making the same. Composite materials have a number of advantages over traditional materials such as steel. As is generally known in the art, a composite material is any material which is manufactured using two or more components having different molecular and physical properties. For example, a common composite object such as a rowing paddle or a snow ski consists of glass fibers (reinforcement) bound together in a plastic material (matrix) such as polyester resin. Both the reinforcement and the matrix have specific functions which contribute desirable attributes to the composite part. The reinforcement carries the loads imposed on the part while the matrix protects and stablizes the reinforcement and holds the shape of the part, thereby channeling the load direction and providing for more efficient load transfer through the reinforcement fibers. The strength of a composite part thus depends on its ability to transfer the load onto the reinforcement component.
The materials which comprise the reinforcement and matrix components of a composite part generally include metals, ceramics, and plastics. Prior art composite parts have been used in many areas, including aerospace and aircraft applications, artificial limbs, marine structures, sports equipment and automotive parts.
The reinforcement component of a composite material can be in the form of a single piece of fabric which may utilize one of many possible weaves and weave angles. However, the reinforcement can also be in the form of a bundle of unidirectional strands of industrial roving material. Embodiments of the present invention can incorporate one or both of these forms of composite reinforcement.
Many different matrix materials can be used to fabricate embodiments of the present invention, including polyester, vinylester, epoxy and phenolic. Moreover, many different reinforcement materials can be used in the present invention, including fiberglass, graphite and KEVLAR.TM.. Those skilled in the pertinent art will understand the use of these and other suitable materials.